my suggestion would be
Measure them and check the diameter
have a look on Maplin.co.uk and see what is available, or pop into halfords.
If you can find something similar, drill out the rivets and put some others
in or use self tapping screws.
Check out eBay, too - you can find some nice car audio stuff there.
Also try www.lowcostcaraudio.co.uk - I got a lovely pair of Clarion
speakers there a while back (and a lot cheaper than the competition).
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